Add new worker for zipping outputs.

Use bakeId to track which outputs are stale.

src/web/ZipWorker.mjs

@@ -0,0 +1,69 @@
+/**
+ * Web Worker to handle zipping the outputs for download.
+ *
+ * @author j433866 [j433866@gmail.com]
+ * @copyright Crown Copyright 2019
+ * @license Apache-2.0
+ */
+
+import zip from "zlibjs/bin/zip.min";
+import Utils from "../core/Utils";
+
+const Zlib = zip.Zlib;
+
+/**
+ * Respond to message from parent thread.
+ */
+self.addEventListener("message", function(e) {
+    const r = e.data;
+    if (!r.hasOwnProperty("outputs")) {
+        log.error("No files were passed to the ZipWorker.");
+        return;
+    }
+    if (!r.hasOwnProperty("filename")) {
+        log.error("No filename was passed to the ZipWorker");
+        return;
+    }
+    if (!r.hasOwnProperty("fileExtension")) {
+        log.error("No file extension was passed to the ZipWorker");
+        return;
+    }
+
+    self.zipFiles(r.outputs, r.filename, r.fileExtension);
+});
+
+self.setOption = function(...args) {};
+
+/**
+ * Compress the files into a zip file and send the zip back
+ * to the OutputWaiter.
+ *
+ * @param {object} outputs
+ * @param {string} filename
+ * @param {string} fileExtension
+ */
+self.zipFiles = function(outputs, filename, fileExtension) {
+    const zip = new Zlib.Zip();
+    const inputNums = Object.keys(outputs);
+
+    for (let i = 0; i < inputNums.length; i++) {
+        const iNum = inputNums[i];
+        const name = Utils.strToByteArray(iNum + fileExtension);
+
+        let output;
+        if (outputs[iNum].data === null) {
+            output = new Uint8Array(0);
+        } else if (typeof outputs[iNum].data.result === "string") {
+            output = new Uint8Array(Utils.strToArrayBuffer(outputs[iNum].data.result));
+        } else {
+            output = new Uint8Array(outputs[iNum].data.result);
+        }
+        zip.addFile(output, {filename: name});
+    }
+
+    const zippedFile = zip.compress();
+    self.postMessage({
+        zippedFile: zippedFile.buffer,
+        filename: filename
+    }, [zippedFile.buffer]);
+};